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A user has done something to his/her profile so that I am not able to find him/her on Facebook.
On the other hand, he/she is able to see my activities.
For example, he/she posted a message on somebody else's wall. Then I posted some comments on the message. Now my friends can see the message along with the comments I posted. But I can neither see the message and nor my comments.
Now, how can I hide myself from that particular user?
If I can't even find him/her, how can I block him/her?
It sounds like the user has blocked you. In that case you cannot see their posts or comments or their profile and also they cannot see your posts or comments or profile. You will not be able to block them except by email address, but they are already effectively blocked so there is nothing you need to do.
Of course, if they have a friend or family member that can see the post then that person may have shown it to them, or if the post was public then they can see it without even logging in.
In the past it was possible for someone to block you so that you could not see them, and then they could log in when you are not on and unblock you so that they could read your comments, and then block you again when they were done. To stop that, Facebook now requires that you wait 48 hours after clicking on "Unblock" before they are actually "unblocked" and also you can't "Block" the same person for 4 days after clicking on the "Unblock". So if someone is blocking and unblocking you, you will have at least 48 hours to see them (and block them if you wish) before they can block you again.
